Scraped surface heat exchangers are a type of mixing apparatus well known in industry. Scraped surface heat exchangers generally feature an outer cylindrical housing tube and a central rotatable drive shaft disposed in the center of the outer housing tube. An annular space is provided between the central drive shaft and the outer housing tube, and material is forced from one end of the scraped surface heat exchanger through the annular space between the shaft and tube.
In general, the material enters the space between the shaft and tube near one end of the scraped surface heat exchanger and travels longitudinally along the scraped surface heat exchanger and exits near the other end of the scraped surface heat exchanger. During this time, the material can be mixed by blades which are attached to, and extend outward from, the central shaft and are rotated by the central shaft. The material may also be subject to temperature gradients as it travels along the scraped surface heat exchanger so that the material is heated or cooled.
The blades are sometimes very generally flat blades which are mounted in a pivotable fashion proximate to the surface of the central drive shaft. The blades extend outward at a relatively narrow angle from the central drive shaft and generally have a tip feature at their distal end which is in sliding contact with the inner surface of the outer housing cylinder as the blades are rotated.
The blades are generally elongated, and typically several blades are provided along the length of the scraped surface heat exchanger to occupy the length of the inner volume of the scraped surface heat exchanger. The blades serve at least some of several functions. For example, the blades can enhance overall mixing of the material as it passes along the inner volume of the scraped surface heat exchanger. The blades also can contact the inner housing of the scraped surface exchanger to in effect scrape material off the inner surface so that it does not build up on the inner surface of the housing cylinder. Further, the blades can add to general flow patterns within the heat exchanger which facilitate temperature transfer from the outer housing of the heat exchanger to the material itself. The outer housing may have a fluid jacket or other heating or cooling source to impart a desired temperature to the outer housing, so that the material can be heated or cooled as it travels through the device.
It has been known to mount the blades to the central driving shaft using so-called “pins”. The pins are individual items that are attached, usually by welding, to the drive shaft surface and have some sort of receptacle area to accept a part of the blade so the blade is pivotally positioned. Two or more pins are usually used on the length of a single blade to pivotally support one blade at the locations of the pins. Previously, the blades have had a cutaway portion forming a hinge shaft or single “attachment beam” that is received in the receptacle slot of the pin to form a hinge configuration.
As noted above, the pins are typically spaced apart longitudinally along the length of the drive shaft, with two or more pins being used for each blade. For simplicity and manufacturing costs reasons, for a given length of heat exchanger it is typically desirable to reduce the number of blades and even further to reduce the number of pins. Thus, it is desirable typically to use relatively longer blades if possible, and it would be desirable to reduce the number of pins for each blade as well.
However, there can be some drawbacks to using longer blades and fewer pins when using the aforementioned prior art pin connection methods. In the prior art, the blades have tended to have relatively short attachment beams due to shear and buckling failure modes. Due to the possibility of flexing of the entire blade between the attachment points, there is a need to place the beams and pins at certain regularly defined intervals.
Another disadvantage with at least some conventional pin and beam attachment systems has been that during assembly of the device, the blade may fall out of its pin receptacle depending on the manufacturing angle of the shaft, pin, and blade.
A different disadvantage with at least some conventional pin and beam attachment systems has been that scraping of the top of the pin against the inside of the outer tube can occur during installation and removal of the drive shaft with the pins and the blades. By way of example, for installation of a drive shaft, it is common practice to first mount each blade into its pivotal connection with its respective pins. Then, the drive shaft itself with all the blades attached (and sometimes flopping loosely to some extent) is gradually manually slid until the tube into its full length runs along the full length of the housing cylinder and then the ends of the drive shaft can be secured to respective bearings and end caps of the housing cylinder. Removal of the drive shaft is through reverse process. Since the clearances between the outside of the drive tube and even more so the outside of the pins, with respect to the inside of the housing tube tend to be very small, it is a very difficult process to manually slide the lengthy drive shaft into the outer housing tube without having the pins contact and to some extent scrape along the inside of the drive tube. Such scraping can be highly undesirable because the surface finish of the inside of the housing tube should generally be kept at a prescribed smoothness if possible. Keeping the inner surface of the housing tube at a prescribed finish helps prevent material from getting caught in surface imperfections on the inside of the tube, which then makes it difficult to clean the inside of the tube during cleanings. Moreover, deep scratches on the inside of the outer housing can lead to premature wear of the blades which are constantly in a rotating sliding contact with the inside of the outer housing tube.
One conventional approach to overcoming this problem has been to insert some form of relatively soft plastic shim or skid between the drive shaft and the housing tube, most particularly at the bottom where gravity would tend to pull the drive shaft down, and then to gently slide the shaft into the tube. The shims can be inconvenient, however, because first of all they need to be present at each assembly and disassembly, and second they may be difficult to insert before a shaft is removed and/or be difficult to remove after a shaft has been installed. Further, in vertical mounted units, the skids can be very difficult or impossible to use.
Accordingly, it would be desirable to have an arrangement where the usually metallic mounting pins do not come in contact with the inner surface of the outer housing tube of the heat exchanger during insertion and removal of the drive shaft with the pins and blades attached.
Accordingly, there is a need in the art for a blade attachment apparatus and method that can overcome the above disadvantages in some instances, at least to some extent, for example by providing desirable support to the blade as well as by providing some degree of locking of the blade to prevent it from falling out during installation, while permitting some pivoting during assembly and operation.